This invention concerns low-pressure metal casting in a multistage recessed sand mold. It further concerns a method and apparatus for feeding the mold impressions with low-pressure casting metal, wherein the feed occurs through a casting chamber, and at each stage at least one intermediate duct begins at the chamber and leads to ingates which connect the duct to the or each impression.
A low-pressure casting method (see, for example, commonly assigned French Patent Nos. 2,295,808; 2,367,566; and 2,556,996) is particularly advantageous, when compared with gravitational casting, for the production of thin-walled metal parts and/or parts having complex shapes and/or parts of large size. In fact, the pressure exerted by the metal, which results from the injection of a gas inside a water-tight cavity containing the molten metal, may be closely and accurately controlled to push the metal into all of the innermost recesses of the impressions.
In conventional techniques, at each stage of the mold, a single, large cross-sectional intermediate duct, or two such ducts positioned diametrically opposite each other, connect the casting chamber to an entire set of ingates belonging to the stage. This method exhibits the following disadvantages linked to the large cross-section of the duct(s):
(1) Strong turbulence is created in the metal flow, thereby promoting erosion of the sand and the occlusion of air bubbles, to the detriment of the integrity of the parts obtained. PA1 (2) It is not possible to cause the metal to rise rapidly to the top of the casting chamber, and filling actually occurs stage by stage, a phenomenon which makes it impossible to profit from all of the advantages of low-pressure casting. PA1 (3) When the pressure is lowered and after the solidification of the ingates, which thus form obturators (see the aforementioned French Patent No. 2,295,808), the metal contained in the intermediate duct, which constitutes a relatively large volume that has cooled appreciably, returns to the casting cavity. During the following casting operation, the cooler metal is the first to rise into the casting chamber, thus adversely affecting the quality of some molded parts. For the same reason, an excessive metal flow is required during each casting. PA1 a) the pressure of the liquid metal is decreased after it is fed into the intermediate ducts, especially when it enters the ingates, PA1 b) a liquid metal flow, adapted to cause the metal to rise above all of the intermediate ducts, is sent through the feed tube, and PA1 c) the feed pressure of the mold is kept constant until the solidification of all of the intermediate ducts, and then the pressure is decreased. PA1 a) the sum of the areas of the ingate sections fed by a single duct is at least equal to the area of the inlet section of the duct, PA1 b) each impression is fed by at least two intermediate ducts extending on either side of the casting chamber, and PA1 c) the sum of the areas of the entry section of the intermediate ducts belonging to each stage is less than 10% of the area of the section of the casting chamber.